which, when allied with ordinary matter, endowed the latter with their
peculiar qualities. The conceptions in each case were properly
mechanical ones _part_ (but not all) _of the time_; for when the
immaterial substances were dissociated from matter, where they had
manifested themselves, no one concerned himself to inquire as to their
whereabouts. They were simply off duty, but could be summoned, like the
genii in the story of Aladdin's Lamp. Now, a mechanical conception of
any phenomenon, or a mechanical explanation of any kind of action, must
be mechanical all the time, in the antecedents as well as the
consequents. Nothing else will do except a miracle.

During the fifty years, from about 1820 to 1870, a somewhat different
kind of explanation of physical events grew up. The interest that was
aroused by the discoveries in all the fields of physical science--in
heat, electricity, magnetism and chemistry--by Faraday, Joule,
Helmholtz, and others, compelled a change of conceptions; for it was
noticed that each special kind of phenomenon was preceded by some other
definite and known kind; as, for instance, that chemical action preceded
electrical currents, that mechanical or electrical activity resulted
from changing magnetism, and so on. As each kind of action was believed
to be due to a special force, there were invented such terms as
mechanical force, electrical force, magnetic, chemical and vital forces,
and these were discovered to be convertible into one another, and the
"doctrine of the correlation of the physical forces" became a common
expression in philosophies of all sorts. By "convertible into one
another," was meant, that whenever any given force appeared, it was at
the expense of some other force; thus, in a battery chemical force was
changed into electrical force; in a magnet, electrical force was changed
into magnetic force, and so on. The idea here was the _transformation of
forces_, and _forces_ were not so clearly defined that one could have a
mechanical idea of just what had happened. That part of the philosophy
was no clearer than that of the imponderables, which had largely dropped
out of mind. The terminology represented an advance in knowledge, but
was lacking in lucidity, for no one knew what a force of any kind was.

The first to discover this and to repudiate the prevailing terminology
were the physiologists, who early announced their disbelief in a vital
force, and their belief that all physi